The New York Times, citing a Western official, said US jets carried out the attack.
Hussein al-Thwadi told Reuters the planes struck at 3.30 a.m. (0130 GMT), hitting a building in the Qasr Talil district in which foreign workers were living.
He said 41 people had been killed and six wounded. The death toll could not immediately be confirmed with other officials.
Sabratha is near the Tunisian border, and it is also one of the areas where Western officials say Daesh (ISIL) militants had some presence as part of their expansion in the North African state.
The gunmen who carried out attacks at a Tunis museum and a beach hotel last year, killing dozens of people, all trained at militant camps in Libya before returning to their home country.
The Western official cited by the New York Times said Friday's airstrikes targeted a senior Tunisian operative linked to both of last year's attacks, which were claimed by Daesh.